In many ecosystems, both natural and agricultural, the productivity of plants is limited by the three primary nutrients: nitrogen, phosphorous and potassium. The most important of these three limiting nutrients is usually nitrogen. Nitrogen sources are often the major components in fertilizers (Hageman and Lambert, I. Corn and Corn Improvement, 3rd ed., Sprague & Dudley, American Society of Agronomy, pp. 431-461, 1988). Since nitrogen is usually the rate-limiting element in plant growth, most field crops have a fundamental dependence on inorganic nitrogenous fertilizer. The nitrogen source in fertilizer is usually ammonium nitrate, potassium nitrate, or urea.
Each year, approximately 85 to 90 million metric tons (MMt) of nitrogenous fertilizers are added to the soil worldwide. This is up from only 1.3 MMt in 1930 and from 10.2 MMt in 1960. It is predicted to increase to 240 MMt by the year 2050 (Tilman et al., Proc. Nat. Acad. Sci. USA. 96: 5995-6000, 1999). It is estimated that 50% to 70% of the applied nitrogen is lost from the plant-soil system. Because NO3-is soluble and not retained by the soil matrix, excess NO3-may leach into the water and be depleted by microorganisms. In fact, most of the applied nitrogen is rapidly depleted by soil microorganisms, leaching, and other factors, rather than being taken up by the plants.
Increased nitrogen utilization efficiency by plants would have a number of beneficial effects. For example, nitrogen utilization efficient plants would be able to grow and yield better than conventional plants in nitrogen poor soils. The use of nitrogen efficient plants would reduce the requirement for the addition of nitrogenous fertilizers to crops. Since fertilizer application accounts for a significant percentage of the costs associated with crop production, such a reduction in fertilizer use would result in a direct monetary savings.
A reduction in fertilizer application would also lessen the environmental damage resulting from extensive nitrogenous fertilizer use. These detrimental effects of nitrogenous fertilizer use on the environment are manifested in increased eutrophication, acid rain, soil acidification, and the greenhouse effect.
Monocots represent a large percentage of the crops grown on the world's 3.7 billion acres of cultivable land. In the United States alone, over 80 million acres of maize, 59 million acres of wheat, 4 million acres of barley and 3 million acres of rice were planted in 2004.
Given the worldwide requirements for monocots and the diminishing fertility of existing fields, it is desirable to generate monocot plants that are able to grow under suboptimal nutrient conditions. One means for accomplishing this goal is to generate monocot plants that can utilize nitrogen more efficiently. Such monocot plants would have the advantage of being able to grow in soils that are poorer in nitrogen, as a result of being able to more efficiently use the nitrogen that is available. Additionally, such monocot plants may demonstrate enhanced productivity in soils that have normal nitrogen levels as well.
Rice is routinely used as the model crop for genetic and physiological studies in other monocot crops including maize, wheat, sugarcane, barley, sorghum, rye and grass. Because of its importance as a model crop, rice was the first crop plant to be sequenced. The International Rice Genome Sequencing Project, a consortium of publicly funded laboratories, completed the sequencing of the rice genome in December 2004. Rice has a small, diploid genome that is well conserved and syntenic across monocots. It is easily transformed and transgenic studies have been performed in rice to study a number of phenotypic traits, including flowering, abiotic stress response, disease resistance, drought tolerance, and morphological development.
Because of the critical importance of nitrogen to plant growth, previous studies have attempted to increase the efficiency of nitrogen utilization in plants using a variety of means. These methods have included conventional breeding programs directed toward the development of plants that are more efficient at nitrogen utilization. Recombinant deoxyribonucleic acid (DNA) and transgenic plant methods have also been employed in an attempt to generate nitrogen efficient plants.
A variety of different genes have been over expressed in dicot plants to increase nitrogen use efficiency with variable results (for review, see Good et al., Trends Plant Sci 9:597-605, 2004). However, monocots and dicots differ from each other in many ways including morphologically, developmentally, metabolically, phenotypically, and genetically. Because of these numerous differences, it would not be predictable that successful whether successful approaches to increase nitrogen utilization efficiency in dicots would necessarily work in monocots.
In the dicot canola, over expression of the enzyme alanine aminotransferase (AlaAT) under the direction of the Brassica turgor gene-26 (also known as antiquitin) promoter elevates AlaAT levels and increases NUE (U.S. Pat. No. 6,084,153). However, whether over expression of AlaAT would increase NUE in monocot plants has not been previously reported.
Increasing NUE within monocot plants is desired within the art.